The Oracle/PLSQL RTRIM function removes all specified characters from the right-hand side of a string.

So it doesn't remove the string_CLUSTER at the end of your string - it removes characters until there is one that isn't _, C, L, U, S, T, E or R. Since your postfixes are R/S/T/U, they also match the rtrim condition, and are removed. As would 123S_SLURTE, for example.